The JDK build fails when SKIP_BOOT_CYCLE=false because of problems with class file version numbers in DirDiff.

Here is the failure:
/opt/jdk/1.6.0/bin/java -XX:-PrintVMOptions -XX:+UnlockDiagnosticVMOptions -XX:-LogVMOutput -client -Xmx896m -Xms128m -XX:PermSize=32m -XX:MaxPermSize=160m -jar /w/hudson/jobs/jdk7.tl.langtools-jdk/workspace/tl/build/solaris-sparc/btjars/dirdiff.jar \
   /w/hudson/jobs/jdk7.tl.langtools-jdk/workspace/tl/build/solaris-sparc/tmp/previous_jdk/jdk1.6.0 /w/hudson/jobs/jdk7.tl.langtools-jdk/workspace/tl/build/solaris-sparc/j2sdk-image | /usr/bin/sort > /w/hudson/jobs/jdk7.tl.langtools-jdk/workspace/tl/build/solaris-sparc/tmp/previous_jdk/previous_jdk_comparison.txt
Exception in thread "main" java.lang.UnsupportedClassVersionError: build/tools/dirdiff/DirDiff : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$000(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
Could not find the main class: build.tools.dirdiff.DirDiff. Program will exit.